Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a tower and a rotor arranged on the tower. The rotor, which typically comprises a hub and a plurality of blades, is set into rotation under the influence of the wind on the blades. Said rotation generates a torque that is normally transmitted through a rotor shaft, either directly or through the use of a gearbox, to a generator. This way, the generator produces electricity which is supplied into the electrical grid.
A wind turbine may comprise rotational systems (e.g. yaw systems or pitch systems) having several motors working together to cause rotation of the system. A yaw system is normally used for orienting the rotor of the wind turbine in the prevailing wind direction. Normally, when the rotor is aligned to the wind direction, the yaw system maintains the position of the rotor. When the rotor is misaligned from the wind direction the yaw system rotates the nacelle to reach an appropriate alignment with the wind.
A yaw system normally performs this rotation of the nacelle by means of a plurality of electric motors with suitable gearboxes for driving gears (pinions) meshing with an annular gear. This annular gear may be attached to the nacelle or to the wind turbine tower. The nacelle can thus be rotated around the tower's longitudinal axis in or out of the wind direction.
Pitch systems are employed for adapting the position of a wind turbine blade to varying wind conditions by rotating the blade along its longitudinal axis. Similarly to the yaw systems explained before, a pitch system of a single blade may comprise a plurality of electric motors for driving actuating gears (pinions) which mesh with an annular gear to set the corresponding blade into rotation. To this end, the annular gear may be provided on the blade and the electric motors and actuators may be mounted on the hub. Alternatively, the annular gear may be provided on the hub and the electric motors and actuators may be mounted on the blade.
An aspect of providing a plurality of motors for a wind turbine rotational system is that local wear on the annular gear may be reduced. Furthermore, when a plurality of motors is used, the motors may be less powerful than if a single motor were used. A consequence of this is that the annular gear may be thinner. Another aspect is that redundancy may be also provided, so that even if one or more motors fail, the wind turbine rotational system can still be operated.
Different approaches for driving the plurality of motors are known. For example, some applications use a common single driver (e.g. electronic converter) for driving all the motors in accordance with common setpoints. An advantage of this approach may be that only one driver is required, which makes the control of the motors rather cheap. However, if this single driver fails, the whole rotational system may become inoperative. Moreover, sharing of loads between the motors may be significantly unbalanced (some could be working as motor and others as generator), which may cause some components of the rotational system to suffer stress, fatigue and/or mechanical wear.
Some other applications use multiple drivers/motors working in parallel without synchronization. This approach is based on having several motors and a driver (e.g. electronic converter) for each motor, the motors being driven (each motor by its dedicated driver) in accordance with setpoints common to all the drivers. Since each motor is driven by its dedicated driver, if one of these drivers fails, only its related motor may become inoperative and not the whole system. The application of this approach is simple and uses relatively cheap components, which makes the control of the motors rather cheap. However, sharing of loads between drivers/motors may be significantly unbalanced (some could be working as motor and others as generator), which may cause some components of the rotational system to suffer stress, fatigue and/or mechanical wear.
Another approach is disclosed by the U.S. Pat. No. 7,944,070B2, which describes a yaw system including a plurality of motors, a plurality of controllers, one controller for each of the motors, and a yaw system controller. Each controller is configured to control the respective motor and coupled to at least one other controller to transmit operation information thereto. The yaw system controller is configured to transmit control information to at least one controller of the plurality of controllers. At least one of the controllers is configured to control the respective motor based on at least one of the control information and the received operation information from the at least one other controller. Some aspects of this system, which is based on communication between controllers, may be that a faster response of the yaw system according to a change of wind conditions can be provided, and that a torque applied on the nacelle can be more equally shared among the plurality of motors.
However, functionalities requiring significant processing capabilities are attributed to the controllers. Moreover, since these functionalities are implemented individually by each controller to achieve desired behaviors of the overall system, a good coordination between said functionalities may be required. Therefore, this system may require complex and expensive controllers with complex and expensive implementations of individual functionalities.